Light guides can be divided into two basic categories. The first, and best known, is solid light guides whose cross-sectional area is composed entirely of a guidance medium. Optical fibres, and larger analogous structures, are of this type. They achieve high efficiency because light is guided by total internal reflection, a non-absorptive process.
The second major category is hollow light guides, which can be more practical because they can guide more light using less material. For example, hollow metallic light guides confine and guide light by means of metallic reflection, which is an absorptive process. More recently, prism light guides have been used to confine and guide light. These are hollow structures which achieve total internal reflection by means of careful arrangement of prismatic surfaces on the exterior of the prism light guide's transparent dielectric wall material. Although prism light guides are more efficient than metallic light guides, they suffer from a serious drawback: they are only able to guide light which falls within a limited range of angles relative to the prismatic surfaces.
It is often possible to collimate the light output of a small light source with a reflector so that essentially all of the reflected light rays satisfy the angular relationship necessary to ensure that essentially all such rays undergo total internal reflection as they propagate along the light guide. But, there are some situations where this is difficult; for example, if the light output of a large diffuse light source is to be coupled into a prism light guide.
In residential lighting applications, for example, it is desirable to couple the light output of comparatively large compact florescent lamps into comparatively small prism light guides; and, in commercial lighting applications, it is desirable to couple light output from radio frequency induction florescent lamps into somewhat larger light guides. In these cases the diffuse light source is too large to allow sufficient collimation by a reflector to ensure that all of the reflected light rays satisfy the angular relationship aforesaid so as to ensure that essentially all such rays undergo total internal reflection as they propagate along the light guide. Consequently, a substantial portion of the light rays emitted by the diffuse light source escape immediately from the light guide in the vicinity of the light source, creating undesirable brightness in that vicinity, and reducing the intensity of the light which remains to be guided along the full extent of the light guide.
The present invention provides a means for coupling the light output of a relatively large diffuse light source into a relatively smaller prism light guide such that most of the light is confined within and guided along the light guide, with only insignificant amounts of light escaping from the guide in the vicinity of the light source.